The crystallization behaviour of an oligomer of hydroxybutyrate containing 24 repeat units has been studied over a wide range of temperature using optical microscopy to measure growth rates and observe morphologies and small angle X-ray scattering to measure crystal thicknesses. Crystals grew with a wide range of thicknesses between E/2 and E, where E is the extended chain length. Preferred crystal thicknesses corresponded to simple fractions of E, which result in a relatively higher proportion of chain folds in the surface. Growth rates peaked at 75°C and were unusually scattered at temperatures corresponding to a change in preferred chain conformation. Spherulites grown at the lower temperatures were banded: as the crystallization temperature was increased the banding disappeared, the shapes of the spherulites became less regular, and a coarser texture associated with reduced branching developed.

High resolution wide angle X-ray scattering has been used to follow changes in crystallinity and lattice parameter which occur on heating hydroxybutyrate oligomers. These effects can be correlated with changes in crystal thickness detected by small angle X-ray scattering. Melt-grown crystals from oligomers with 24 and 32 repeat units that initially contain once folded chains transform during slow heating to produce extended chain crystals. This chain unfolding occurs via a process of partial melting and recrystallization and is accompanied by an expansion in the crystal lattice of approximately 0.2% in the (110) fold direction. The presence of a benzyl protecting group on one end of the chain does not affect the transformation temperature but reduces the (020) and (110) lattice spacings very slightly, as well as increasing the rate of transformation. In all cases the stable crystal forms display specific crystal thicknesses simply related to the extended chain length, but the intermediate stages of thickening vary between samples.

Exact length hydroxybutyrate oligomers containing 24 and 32 repeat units have been prepared and their crystallization behaviour and crystal morphology observed.The oligomers form crystals from dilute solution and from the melt that have similar overall morphologies to crystals of the polymer, poly(hydroxybutyrate). Electron diffraction indicates that the chains are closely perpendicular to the basal planes of the crystals.Wide angle X-ray diffraction suggests that all crystals, no matter what their thickness, have a high degree of crystallinity and the same crystal structure.The crystals have a range of thicknesses, as measured by small angle X-ray diffraction. The most common thicknesses correspond to the extended stem length and to one half or, surprisingly, two thirds of that value. A model is proposed for the non-integer fraction crystals in which half of the chain ends are incorporated in the crystals themselves. (C) 2004 Elsevier Ltd. All rights reserved.

Synchrotron X-ray radiation has been used in situ to follow the crystallization of a hydroxybutyrate oligomer containing 32 repeat units from dilute solution in propylene carbonate, and to study chain unfolding during heating in oligomers with 24 and 32 repeat units. A discontinuity in growth rate occurs at 36–37°C for the 32-mer: crystals grown below this temperature contain folded chains, which transform during heating through a process of partial dissolution and re-crystallization to form extended chain crystals. Crystals grown above the transition temperature contain extended chains that do not rearrange during heating. A similar change happens in the 24-mer between 35 and 40°C. Thermal expansion in the (110) lattice direction (the fold direction) was in the range 5.3+/−0.3×10−5nm°C−1, with an additional discontinuous increase in 0.001nm accompanying chain unfolding.